1. Field of the Invention
This invention relates generally to phase-change memory. More particularly, this present invention relates to methods and apparatus for determining the state of phase-change memory cells.
2. Description of Related Art
Phase-change memory (PCM) is a new, non-volatile solid-state memory technology that exploits the reversible switching of certain chalcogenide materials having at least two states with different electrical conductivity. PCM is fast, has good retention and endurance properties, and has been shown to scale to the future lithography nodes.
In single-level cell (SLC) PCM devices, the fundamental storage unit (the “cell”) can store one bit of binary information. The cell can be set to one state out of two states, which include crystalline and amorphous, by application of heat. In the amorphous state, which represents binary 0, the electrical resistance of the cell is high. When heated to a temperature above its crystallization point and then cooled, the chalcogenide material is transformed into an electrically-conductive crystalline state. This low-resistance state represents binary 1. If the cell is then heated to a high temperature, above the chalcogenide melting point, the chalcogenide material reverts back to its amorphous state on rapid cooling.
In multilevel-cell (MLC) PCM devices, a memory cell can be set to s different states, where s>2, in order to permit storage of more than one bit per cell. MLC operation is achieved by exploiting partially-amorphous states of the PCM cell. Different cell states are set by varying the size of the amorphous region within the chalcogenide material. This varies cell resistance. Thus, each cell state corresponds to a different amorphous volume which in turn corresponds to a different resistance level.
To write data to a PCM cell, a voltage or current pulse is applied to the cell to heat the chalcogenide material to an appropriate temperature to induce the desired cell-state on cooling. Reading of PCM cells is performed using cell resistance as a metric for cell-state.
The resistance of a cell can be measured in various ways, usually by biasing the cell at a certain constant voltage level and measuring the current that flows through it. U.S. Pat. No. 7,505,334 B1 discloses an alternative method whereby cell resistance is detected from the discharge time of an RC (resistor-capacitor) circuit in which the cell is the resistor. However measured, the resulting resistance indicates cell-state according to the predefined correspondence between resistance levels and cell-states.
The resistance measurement for a read operation is performed in the sub-threshold region of the current-versus-voltage (I/V) characteristic of the cell, i.e. in the region below the threshold switching voltage at which a change in cell-state can occur. Since the threshold switching occurs at a fixed electric field, the states which correspond to low amorphous size undergo threshold switching at lower bias voltages. A low, and hence safe, bias voltage is therefore used for reading all cells. In this low-field region, all cells can be read without affecting cell-state.